Infiltration via submerged drains reduces the subsidence with 50 -70% depending on the ditch water level. Combination of submerged drains with a high ditchwater level of about 20 cm minus surface level reduces subsidence to less than 1 mm per year.

For more information about this RECARE experiment, please contact Jan van den Akker This email address is being protected from spambots. You need JavaScript enabled to view it.

Further information about the case study activities in Dutch can be found on the Stakeholder Platform here.

Geographical description

Veenweidegebied (51°57' N, 4°43' E, -1.7 m a.s.l.) is situated in the centre of the peat land area, Lopikerwaard (27 km2) and has been in permanent pasture for centuries. Mean annual temperature is 10.2 °C and mean annual precipitation is 754 mm. Near the rivers, these peat soils are covered with a 20-40 cm thick layer of heavy clay. Peat depth is 6-8 meters and the distance between ditches is about 30-40 meters representing about 15% of total area is open water. Ditchwater levels are 50 – 60 cm below surface level. The area is typical of agricultural use for about 100,000 ha of peat soil in the Western part of the Netherlands.

Main soil threat

About 9% of the area of the Netherlands is covered by peat soils (about 290,000 ha), mainly drained and in use for dairy farming. Peat soils in the densely inhabited western part of the Netherlands are valued as an open landscape with a rich cultural history. The major part of peat soils in the western part of the Netherlands is in use as permanent pasture with ditchwater levels up to 60 cm below the surface. Organic soils above groundwater level are exposed to the air and decompose (oxidation). This causes a subsidence of 8-12 mm.y-1 and a CO2 emission of 18-27 tons.ha-1.y-1. We calculated an emission of 4.25 Mt CO2.y-1 for the agricultural peat soils in the Netherlands.

This is 2.5% of the national anthropological CO2 emission of the Netherlands and equivalent with the CO2 emission of 1.7 million cars. The degradation of peat soils results also in emission of nitrogen (N) and phosphorus (P) and sulphate (SO4) towards surface waters and an extra total Dutch N2O emission of 0.5 Mt CO2 equivalent y-1. Periodically in peat areas, ditchwater levels and thus groundwater levels must be adapted to the subsidence. As a consequence, once submersed wooden foundation piles are now being exposed to oxygen and starting to rot, causing damage to infrastructure and buildings. Because subsidence is not the same everywhere, water management becomes ever more complex and expensive. Many wetlands become difficult to preserve because subsidence of adjacent drained agricultural land results in drainage of the wetlands towards the surrounding lower elevation agricultural lands. In time, with rising sea levels in a nation largely under sea level, it is not wise to allow continued subsidence rates of 1 cm.y-1. It should be stressed that due to climate change (higher temperatures and longer, dryer periods) subsidence rates will be doubled by the end of this century. Raising ditchwater levels seems a simple way to reduce oxidation of peat soils, however, ditchwater levels of 20 cm below the surface in summer are needed to halve subsidence and GHG emissions. This makes a viable dairy farming impossible and dairy farmers very strongly oppose this kind of solution.

As an acceptable alternative ALTERRA started research on infiltration of ditchwater via submerged drains to raise groundwater levels in summer to conserve peat land. Model simulations and first results of field experiments show that this technique can halve subsidence and the resulting CO2 emissions. Also, water quality is expected to improve, however, model studies show that a disadvantage might be that due to the extra water infiltration via drains, the water need in summer will increase. Infiltration via submerged drains is a promising technique to halve peatsoil degradation, while it is also acceptable for a wide range of stakeholders. First results are promising, however, more proof in praxis is needed before this technique can be promoted and introduced on a scale of tens to thousands of hectares of peat land: can the groundwater level be raised upon in almost all ditchwater levels in very dry summers? Will subsidence and GHG emissions be halved? Will ditchwater quality improve? Is intelligent water management possible to reduce the extra water needs in dry summers to an acceptable level? Are the (economic) advantages for farmers substantial enough, so they will invest in submerged drains? Are there possibilities to earn Carbon Credits to cover the costs (and more!) etc.

Other soil threats

Peat soils are sometimes completely saturated for long periods in winter, thus posing a direct threat to soil biota including earthworms. On the other hand this is typical for these peat grass lands and is a consequence of the aim to reduce subsidence and peat oxidation by limiting drainage depths. This wet period in winter and spring is also important for other parts of the ecosystem (e.g. meadow birds). It should be noted that the peat meadow area in the Netherlands is extremely important for the breeding success of some meadow birds. For instance 90 % of the population of the black-tailed godwit (Limosa limosa) in North-Western Europe is breeding in the Netherlands. Therefore soil biodiversity and relevant ecosystem services typical for peat meadow soils may be at risk when peat soils disappear by oxidation and turn into mineral soils or when these peat soils are rewetted to turn them into nature.

The distance between drainage ditches is about 30 – 40 m and about 15% of the total area is open water. Ditchwater levels are 50 – 60 cm below soil surface. The Case Study area is representative of about 100,000 ha of peat soil in agricultural use in the Western part of the Netherlands called “veenweidegebied” (peat meadow area).

Natural Environment

Geology and Soils The Krimpenerwaard is part of the river delta area of the rivers Rhine, Meuse and Scheldt behind a row of dunes along the shore of the North Sea. The low area behind the dunes used to be covered by swamps and lakes-below.

Peat formation and degradation in The Netherlands up to the Middle Ages (BP = Before Present). In the late Middle Ages people started to drain and reclaim peat soils

From time to time there were intrusions from the sea. Thick layers of fenpeat formed because of the slow subsidence by tectonic movement and sea level rise. The whole western and Northern part of the Netherlands was covered by fenpeat. After a while, raised bogs started to grow covering the fen peats. Later on, large areas of peat were lost by intrusions of the sea. At the end of the Middle Ages, people started to exploit and drain large areas of peatland for agricultural purposes and, later on, for fuel and salt extraction. The Krimpenerwaard also had a raised bog, but nowadays the sphagnum moss peat has completely disappeared. The Krimpenerwaard is surrounded by rivers. Due to the subsidence of the drained peat and the sea level rise, higher river water levels caused the Krimpenerwaard to be flooded annually, either near the rivers or completely. During floods, clay was deposited on the peat soils (see above), thus reducing the oxidation and subsidence. In the 12th and 13th century BC the Krimpenerwaard was surrounded by dikes and became a polder. In the first centuries thereafter the land subsided slowly below the river water level and the ditchwater had to be pumped out mechanically, first using windmills, then steam engines and later electricity driven pumps. Nowadays the surface level is about 1.70 m below m.s.l.The soil is classified as Terric Histosol (FAO classification). The top soil consists of clayey decomposed peat on top of about four meters wood-sedge and reed-sedge peat. Soil properties are presented in the tables below. Soil properties and water levels are typical for large areas in the peat meadow regions in the western part of The Netherlands.

Soil sampling. In front the monitoring ditch divided by a dam in a part with submerged drains (right) and a part without submerged drains (left).

Climate The climate in this region is Atlantic and temperate. Mean annual temperature is 10.2 oC and mean annual precipitation 754 mm.

Left: Average annual; right: mean monthly precipitation and temperature at the weather station of Rotterdam for 1981 – 2010.

Land Use The Case Study is entirely covered by grassland with Perennial ryegrass (Lolium Perenne) as the dominant species and is used for intensive dairy farming.

Hydrogeology The mean deepest groundwater level taking place at the end of summer is about 65 cm below the average soil surface level. The mean highest groundwater level taking place in winter is about 15 cm below the average soil surface level. Ditchwater levels, controlled by the Water Board, are about 50 cm below the mean soil surface level in winter and summer.

Drivers and pressures

Human activities with drainage and cultivation are important drivers for processes leading to decline in organic matter. The human activities are highly influenced/driven by the socioeconomic conditions supporting cultivation. Climate and climate change are also major drivers of peatland degradation. In the period 1950 – 2012 the temperature increased about 1.4 oC, which is about twice as much as the average increase worldwide. This faster rise is caused by an increase of western winds and decrease of aerosols in the atmosphere of The Netherlands and less cloud cover in case of winds from the east and south. The precipitation in the coastal area is also increasing, due to higher water temperatures of the North Sea. Other issues playing an important role are water quality and water use to keep the peat wet and protection of meadow birds.

Status of soil threat

Subsidence is nowadays about 1 cm/y. This means an annual loss per hectare of about 12.2 ton OM or about 22.6 ton CO2 or 6.7 ton C (Figure 11.6). At the Case Study area, peat depth is about 4 m. Several meters of peat have already been lost by subsidence and nowadays the surface level is about 1.70 m below average sea level. It is expected that in one century the subsidence due to peat oxidation in the western part of the Netherlands will be 1.5 m. These peat areas are already situated well below sea level and the subsidence will more than double the impact of sea level rise. It is clear that adaptation strategies are needed to increase the resilience of peat soils to climate change. Further subsidence and associated lowering of the ditchwater levels will also result in an upward seepage of nutrient rich groundwater and in some cases even salt water. The surface water levels of lakes, nature reserve parks and the so-called high-water ditches along houses are kept constant. The purpose of high-water ditches is to keep the groundwater levels around the wooden foundation piles high enough to prevent rot. In time the agricultural land will subside below the constant water level so dikes are required, along with their respective maintenance. The stability of these dikes is also questionable since they have been founded on peat soil. There are quite big differences in the rate of subsidence in the Krimpenerwaard because subsidence depends on ditchwater levels and thickness of an eventually existing clay layer on top of the peat. This results in an increasingly complex water management system and frequent adaptation of the water management infrastructure to changing conditions is required. The same issues arise to a certain extent regarding road infrastructure.

CO2-emissions of peat areas in The Netherlands in ton/ha/year and soil map of the Krimpenerwaard.

WOCAT Maps

Maps on the current state of land use, soil degradation and soil conservation in the case study area have been produced using the WOCAT (World Overview of Conservation Approaches and Technologies) methodology

The steps of this process are as follows:

1) The area to be mapped is divided into distinctive land use systems (LUS). 2) The team gathers the necessary data on soil degradation and conservation for each LUS using a standardised questionnaire, in close consultation with local land users, and supported where possible by remote sensing or field data. 3) For each LUS, the soil degradation type, extent, degree, impact on ecosystem services, direct and indirect causes of degradation, as well as all soil conservation practices, are determined. 4) Once collected, the data is entered in the on-line WOCAT-QM Mapping Database from which various maps can be generated.

Following the principles of all WOCAT questionnaires, the collected data are largely qualitative, based on expert opinion and consultation of land users. This allows a rapid and broad spatial assessment of soil degradation and conservation/SLM, including information on the causes and impacts of degradation and soil conservation on ecosystem services.

More details about the methodology used to produce these maps and their interpretation can be found here.

Land Use (click on maps to expand)

Degradation

The degree of degradation reflects the intensity of the degradation process, whilst the rate of degradation indicates the trend of degradation over a recent period of time (approximately 10 years).

Conservation measures

The "effectiveness" of conservation is defined in terms of how much it reduces the degree of degradation, or how well it is preventing degradation. The Effectiveness trend indicates whether over time a technology has increased in effectiveness.

In fact peat soils are not suited as platform for structures and infra-structure. Subsidence by peat oxidation and associated lowering of groundwater levels increases the problem. In the long run the risk of flooding is getting a major problem.

Increasing damage and costs in time. Increasing risk of flooding.

Source of raw materials

Not in the Krimpenerwaard

Not in the Krimpenerwaard

Table 11.3: Effects of oxidation of peat soils on soil functions.

Administrative and socio-economic setting

Almost all peat soils in agricultural use are privately owned. All water management in the area is regulated by the District Water Board “Schieland en de Krimpenerwaard”. The Schieland en Krimpenerwaard control area stretches from Rotterdam, Zoetermeer to Schoonhoven. Within that area, the Water Board is responsible for flood control (maintenance of the dikes), water quantity (correct water levels), surface water quality and treatment of urban wastewater. Since 2015, five small municipalities are merged forming together the municipality Krimpenerwaard. The municipality of Krimpenerwaard is covering the region Krimpenerwaard except the municipality Krimpen aan de IJssel (see map above). The municipality Krimpenerwaard has 54,142 inhabitants - below and an area of more than 160 km2 and is mainly countryside with small communities. The main economical basis is dairy farming and to a smaller extent recreation and tourism. The municipality Krimpen aan de IJssel has 28,930 inhabitants (31 March 2015) and an area of 8.93 km2 and is mainly oriented to the region of Rotterdam.

Left: population in municipality Krimpenerwaard; right: GDP per capita trends for the Netherlands and the Euro Area.

Management options

It is clear that adaptation strategies are needed to increase the resilience of peat soils to climate change. Raising groundwater levels through the use of effective and intelligent water management is a key factor to preserve peat soils and reduce CO2 emissions and subsidence.

In the Case Study we test and demonstrate the use of submerged drains to infiltrate ditchwater into the parcel in order to raise summer groundwater levels and conserve peat soils (see below). The set-up of the experimental site is presented below. Ditchwater and shallow and deep groundwater levels are measured every 8 hours. Water is pumped in and out at the North and South dam to keep the ditchwater level constant at 50 cm below the mean soil surface level. These water fluxes are measured. During two years water quality is measured every two weeks. To determine the subsidence rate of soil surface levels, altitudes are measured every two meters, in three cross sections, at each measuring field during spring. The measure of submerged drains is expect to stall the current decline in the altitude surface levels due to peat subsidence (1 cm/y) by at least 50%. Periods of complete saturation will also become shorted therefore having beneficial effects to soil and ecosystembiodiversity and the use as permanent pasture for dairy farming.

Prevention of deep groundwater levels in peat soils by infiltration of ditch water via submerged drains. This will halve the oxidation of peat soil. Note: a rise of the mean deepest groundwater level with about 20 cm will reduce the subsidence with 5 mm and the CO2-emission with 11 tonne CO2 /ha/year (Van den Akker et al., 2010, 2012).

Sketch of the set-up of the experimental site. Width of the parcels is about 30 m, length of each section (with and without submerged drains) is about 145 m. Distance between the drains is 6 m.

Stakeholder involvement

Relevant end-users and local stakeholder groups include;

Land-owners in the area (private farmers)

Water boards

Provinces and ministries

LTO (Dutch farmers association)

Agricultural societies and advisory organisations

Researchers

The general public

There is an active network group of farmers in the area dealing with submerged drains. The network group also includes PPP Agro Advice (SME), VIC (Peat meadow Innovation Centre) and DLO. There is also an existing stakeholders group on submerged drains including the provinces of South-Holland and Utrecht, six water boards, LTO (Dutch farmers association) and the Ministry of Economics and Agriculture, DLO and VIC. Both stakeholder groups are involved and the Case Study and the project will be carried out in close collaboration with the stakeholders.